Q: Custom Desktop Picture Doesn't Stay After Reboot

I am running a MacBook Pro with an attached monitor in a dual screen set up. The external monitor is the "primary" monitor with the MacBook being the secondary, I've had this setup going for years, but have NEVER wanted to change the wallpaper.

When I set a picture as the desktop background I set it to both screens, but when I reboot the default Yosemite wallpaper is on the primary display and the custom one is only on the secondary. I've tried a variety of fixes. I've added the folder with the picture to the Folders List in the Desktop & Screensaver settings. I've even moved the pic to my official "Pictures" folder for my account.

Any idea as why this would happen and a way to fix it? I love the default picture, but it's getting a little grey.

OSX 10.10.5

Mid 2012 15-Inch MacBook Pro